Depending on their type, such housings and locking elements are used for the connection of various elements in cases where such elements are to be connected fixedly to one another, for example in general mechanical engineering (e.g. in shaft-hub connections, as are known from DE-A-42 09 153, or in other connections, e.g. in screw connections where two structural parts are to be locked fixedly to one another).
A specific field of application of such connections is the field of medical engineering, for example the field of traumatology, where it is not uncommon to perform fixation of bone segments, for example in fractures. For optimal treatment of the fractures, the individual bone segments have to be repositioned as exactly as possible and have to be fixed in the desired position with the aid of a bone plate and bone screws, to ensure that treatment is as successful as possible.
For successful treatment, it is important, after they have been fixed, that the bone segments remain stable with the aid of the bone plate; for example, a bone screw must not come loose from the bone plate, as can happen, for example, in rare cases of undesired loading of a treated fracture. In such cases, the bone segments may then assume a position different than the desired position, which may reduce the degree of success of the treatment.
WO-A-00/66012 has already proposed a bone plate in which an engagement contour made in the plate aperture has a plurality of horizontally and radially extending contour valleys and, adjacent to these, contour peaks. However, the engagement contour does not extend completely round the plate aperture, and instead runs out at two horizontally opposite ends. There, uncontoured wall areas remain in the plate aperture, as a result of which the plate aperture has an oblong hole shape. The associated bone screw has a bone thread on its shank, and above the bone thread, but below the screw head, there is an additional locking thread. While the bone thread is screwed in the usual manner into the bone segment, the locking thread engages in the engagement contour of the plate aperture in the final phase of screwing-in of the bone screw. Since the locking thread with its helical course and its pitch does not complement the engagement contour in the plate aperture, this causes deformation of both parts and, therefore, a strong connection of bone plate and bone screw and, although this connection can in principle be released again, automatic loosening of the screw is reliably avoided.
For example, when two bone segments, namely a first bone segment on one side of a fracture and a second bone segment on the other side of the fracture, are brought into a desired position relative to one another, a bone screw is screwed into each of the two bone segments via the plate aperture of the bone plate, and the bone screws are locked in the plate aperture of the bone plate, so that the bone segments remain in the desired position after locking. The stable locking in the plate aperture also ensures, inter alia, that the bone plate does not have to lie on the bone, and instead, for example, can also be arranged in a stable manner at a short distance from the bone. This may be of advantage, because the periosteum ensures better blood circulation and nutrition of the bone, and an intact periosteum thus promotes the healing process. This very functionally reliable bone plate described in WO-A-00/66012 allows the screw to be introduced with a slight deviation from the perpendicular axis of screwing; the perpendicular axis of the plate aperture and the actual direction of screwing can therefore enclose a small angle between each other. However, such a deviation of the actual axis of screwing from the perpendicular axis of the plate aperture (tilting of the actual axis of screwing relative to the perpendicular axis of the plate aperture) can only be present in the direction of the longer axis of the oblong plate aperture (that is to say in the direction of the two ends of the plate aperture with the uncontoured wall areas), and it is possible only within a relatively small angle range.
In many cases, however, it would be advantageous for the operating surgeon to be able to fit a bone screw as freely as possible in respect of direction, for example if a fracture to be treated lies outside the access route, or if the amount of available bone substance is such that the screw would be best fitted in a specific direction. It should further be noted that, in many cases, subsequent repositioning of a bone segment may be desirable. Although this is possible in principle in the bone plate known from WO-A-00/66012, such repositioning is more difficult because of the deformation of locking thread and engagement contour. However, in complicated fractures in particular, with several bone segments, in some cases with small bone segments, for example in the region of the hand, it is often very important, for optimal positioning of the individual bone segments, that the locking can be released again without damaging the screw and/or the plate and that individual bone segments are able to a certain extent to be positioned differently.